Rheology of Earth Materials :

Closing the gap between timescales in the laboratory and in the mantle

13 July 2020: Is the Earth’s transition zone deforming like the upper mantle?

Solid-state convection of the Earth’s mantle involves lithospheric subduction and the ascend of plumes from and towards the Earth’s surface. In the upper mantle, this relies strongly on the mechanical behavior of its main constituent, Mg2SiO4 olivine, in the ductile regime. Because the first ~300 km of the upper mantle is characterized by a significant seismic anisotropy, it is generally believed that the Weertman type of dislocation creep - a deformation mechanism inducing lattice rotation and crystallographic preferred orientations (CPO) in elastically anisotropic minerals as olivine - contributes to its overall deformation. Indeed, recent dislocation dynamics (DD) simulations (Boioli et al. 2015) have shown that intracrystalline plasticity of Mg2SiO4 olivine under relevant upper mantle conditions is accommodated by Weertman creep, where the climb of dislocations enables to recover dislocation junctions, allowing plastic strain to be efficiently produced by dislocation glide.

Entering the mantle transition zone beyond ~410 km depth, olivine transforms first into its high-P polymorph wadsleyite and at ~520 km into ringwoodite. Previous work on numerical modeling of thermally activated dislocation glide by Ritterbex and co-authors (Phys. Earth Planet. Int. 2016; Am. Mineral. 2017) have shown that in contrast to olivine at upper mantle conditions, even those dislocations belonging to the “easiest” slip systems in wadsleyite and ringwoodite experience substantial lattice friction inhibiting their glide motion.

Figure 1.  Ratio of the glide versus climb mobilities of (a) the ½<111>{101} slip system in wadsleyite at 15 GPa and (b) the ½ <110>{110} slip system in ringwoodite at 20 GPa.

 

Combining the previously derived glide mobilities together with experimental diffusion data, we demonstrate in this paper that dislocation climb velocities exceed those of glide in the high-P polymorphs of olivine at P,T conditions of the transition zone, inducing a transition in deformation mechanism in the dislocation creep regime from Weertman creep to pure climb creep at geologic relevant stresses (Fig. 1).

Based on analytical plasticity modeling and constrained by diffusion data from experiments, we finally report the contribution of steady-state plastic deformation of the main transition zone minerals wadsleyite, ringwoodite and majorite garnet by both diffusion- and dislocation creep mechanisms (Fig. 2).

Figure 2. Deformation mechanism maps of (a) wadsleyite at 15 GPa and 1500 K, (b) ringwoodite at 20 GPa and 1700 K and (c) majorite garnet at 18 GPa and 1600 K.

 

We show that intracrystalline plasticity of wadsleyite, ringwoodite and majorite garnet by pure climb creep at geologic stresses would lead to an equiviscous transition zone of 1021±1 Pa.s if the grain size is ~0.1 mm or larger, without the need of diffusion-related hydrolytic weakening, matching well the available geodetic observations. Since pure climb creep does not induce lattice rotation and cannot produce CPO, the latter is compatible with the relative seismic isotropy of the bulk transition zone compared to the upper mantle. Nevertheless, our model results also predict that CPO is able to develop along with stress concentrations by the activation of Weertman creep, for example in corner flows around cold subducting slabs, something that could induce an increase in subduction resistance, explaining why some slabs stall at the base of the transition zone. On the other hand, viscosity reductions are predicted if grains are smaller than ~0.1 mm when the transition zone silicates are deforming in the diffusion creep regime, which might potentially influence flow dynamics in the interior of cold subducting slabs or across phase transitions (Fig. 3). Future incorporation of these deformation mechanisms as a function of grain size in geodynamic convection models should enhance understanding of the mass transfer between the upper and lower mantle.

 

Figure 3. Illustration of the dominant intracrystalline deformation mechanisms expected in wadsleyite (Wd), ringwoodite (Rw) and majorite garnet (Mj) across the mantle transition zone compared to those of olivine in the upper mantle.

References:

  • F. Boioli, Ph. Carrez, P. Cordier, B. Devincre & M. Marquille, (2015) Modeling the creep properties of olivine by 2.5-D dislocation dynamics simulations. Physical Review B, 92(1) 014115, doi: 10.1103/PhysRevB.92.014115.
  • S. Ritterbex, Ph. Carrez, K. Gouriet & P. Cordier (2015) Modeling dislocation glide in Mg2SiO4 ringwoodite: Towards rheology under transition zone conditions. Physics of the Earth and Planetary Interiors, 248, 20-29. doi.org/10.1016/j.pepi.2015.09.001.
  • S. Ritterbex, Ph. Carrez & P. Cordier (2016) Modeling dislocation glide and lattice friction in Mg2SiO4 wadsleyite in conditions of the Earth’s transition zone. Americal Mineralogist, 101(9), 2085-2094, doi: 10.2138/am-2016-5578CCBYNCND (selected by the Editors as “Notable paper”)

To learn more:

S. Ritterbex, Ph. Carrez, P. Cordier, 2020. Deformation across the mantle transition zone: A theoretical mineral physics view. Earth and Planetary Science Letters 547, 116438.